The present invention relates generally to electrical plugs, and more specifically to such a plug of simplified design for use in hazardous locations.
Electrical plugs used to deliver electrical current from an energized receptacle to an electrical device, such as a motor, piece of machinery, etc., are well known in the art. Generally, they consist of a cable or cord containing two or more conductor wires that are attached to an equal number of terminals fixed in a support housing. Once the plug is inserted into the receptacle, current travels through the terminals and conductor wires to the electrical device.
Because it is important to prevent migration of moisture into the plug housing that could interfere with the electrical current and cause safety hazards, these plugs are generally provided with one or more sealing gaskets made from an elastomeric material. Portions of the housing or an electrical connector cooperate to exert compressive forces against the gasket to enhance the moisture-resistant seal. Examples of plug housings or electrical connectors using such gasket seals include U.S. Pat. Nos. 5,108,303 issued to Maeda, et al.; 4,917,620 issued to Samejima et al.; 5,114,359 issued to Chishima et al.; Reissue No. 32,787 issued to Gallusser et al.; 4,857,006 issued to Linyeav et al.; 4,676,575 issued to Denlinger et al.; and 3,430,187 issued to deMan et al.
While the conductive terminals have been traditionally connected to the conductor wires through the face of the plug to form a so-called "live-front" plug, the recent trend within the industry has been toward the much safer "dead-front" design in which the terminals are wired from the back of the insulative plug facing, which contains slots through which only the terminal prongs protrude. DeMan, as well as U.S. Pat. Nos. 4,563,049 issued to Thibeault, 4,284,318 issued to Wiley et al.; and 4,178,056 issued to Lee, illustrate this dead-front plug design.
Because of pulling forces frequently applied to the conductor wires due to the operator or operating electrical device, the wires may become disengaged from one or more of the plug terminals to create risks of a short circuit or injury to the operator. Therefore, the plug must include a means that mechanically grips the exterior surfaces of the conductor wires in order to relieve the strain imposed upon individual wires at the terminal junctions. Several different clamping devices are used in the trade to achieve this objective. In deMan, for instance, an end cap and intermediate housing section cooperate to define a chamber in which a compressed gasket engages the conductor side wall to provide a mechanical grip. In Lee, a set screw biases a metal prong against the conductor sidewall. The plug disclosed by Thibeault includes two cord grips protruding from the plug housing that are biased by a cooperating end cap into engagement with the conductor sidewall. Finally, the plugs taught by Wiley and Clark include two clamp halves that are secured around the conductor by means of screws.
As a measure of added safety, the plug housing portions are generally made from a non-conductive material like plastic with only the terminal prongs made from metal, as taught by Thibeault, Wiley, Lee, and deMan. In the case of accidental separation of the conductor wires from the terminal prongs, such a design prevents short circuits and transmission of current to a metal housing that could cause substantial safety hazards.
For plugs used in hazardous locations where gases, dust, and other ambient particulate matter can explode when ignited, it is important to isolate arc-producing devices like plugs so that ignition of such matter within the plug housing will not spread to the equally explosive environment outside the housing. Therefore, the housing usually must be made from a material like a thick-wall cast aluminum in order to contain any such explosions. The risk of a short circuit should a conductor wire separate from a terminal is alleviated by interposing a plastic-insulating liner between the metal housing and exposed conductor wires. See, e.g., U.S. Pat. No. 5,135,404 issued to Clark et al.
An alternative prior art plug design 10 sold by Appleton Electric for use in hazardous locations is shown in FIGS. 1-3. It comprises an upper housing 12 containing a cylindrical side wall 14 and an end wall 15 defining a plug chamber 16. Located along the interior surface of side wall 14 is shoulder 18 and two ears 20 containing threaded bores 22 therein. End wall 15 of upper housing 12 includes a passageway 26 cut therethrough, and two posts 28 extending therefrom with semicircular niches 30 formed therein, one such post 28 being shown in FIG. 2.
First and second clamp halves 32 and 34, respectively, feature a plurality of elliptically-shaped teeth 36 along the inner surface that are offset with respect to each other when the clamp halves are fastened together by means of screws 38 and threaded receptacles 40 to secure the conductor wires (not shown). Clamp halves 32 and 34, and the conductor wires, in turn, are retained by upper housing 12 by means of engagement of niches 30 by screws 38.
Conductor wire clamps 32 and 34 are covered by lower housing 44 that comprises cylindrical side wall 46 having two annular bores 48 therethrough, and end wall 50 having a circular hole 52 formed therein. An elastomeric grommet 54 with an annular niche 56 along its exterior perimeter surface is pressed into hole 52 of lower housing end wall 50 so that annular niche 56 is engaged by end wall 50.
Lower housing 44 is secured to upper housing 12 with an elastomeric gasket 60 positioned therebetween by means of screws 62 that pass through annular bores 48 in lower housing 44, and enter threaded pockets (not shown) in ears 64 protruding from upper housing 12. A hole in grommet 60 permits the conductor wires to pass into plug chamber 16, while minimizing moisture penetration therein.
A nonconductive plug base 68 through which a plurality of terminal prongs 70 made from a conductive material protrude is secured to the open end of upper housing 12 by means of screws 72 that engage the threaded surfaces of bores 22. Plug base 68 actually bears against shoulder 18 of upper hosing 12, compressing elastomeric gasket 74 therebetween to provide a moisture-resistant seal. Connected to the other ends of terminal prongs 70 are fasteners 76 for accepting the individual wires of the conductor. These fasteners could comprise lugs and set screws. Partition wall 78 extends from the bottom surface of plug base 68 to separate the conductor wires at the point where they are secured to terminal fasteners 76 in order to reduce the possibility of short circuits. Finally, a sleeve 80 made from an insulative material like plastic lines the interior surface of upper housing side walls 14 in order to prevent short circuiting between the conductor wires and the housing walls that are made from aluminum material to render plug 10 suitable for use in hazardous locations.
In this manner, plug 10 will: communicate electrical current from terminal prongs 70 and the conductor wires to an associated motor, machine, etc.; securely hold the conductor wires with respect to the plug terminals 70 to reduce the likelihood of separation of the wires from terminal fasteners 76 should they be pulled; prevent short circuits between the conductor wires and housing side wall 14 by means of insulating sleeve 80; and reduce moisture penetration into the plug housing by means of elastomeric gaskets 54, 60, and 74.
However, the design of plug 10 suffers from several disadvantages. First, shoulder 18 must be cast into upper housing 12 to provide a bearing surface for gasket 74 and plug base 68. Second, separation of clamp halves 32, 34 from upper housing 12 requires the presence of lower housing 44 to protect the clamped conductor wire in hazardous locations. Third, plug 10 requires a large number of intricately shaped parts that must be machined perfectly to permit cooperative engagement. Fourth, the label plate for plug 10 needs to be screwed to the exterior surface of upper housing side wall 14 by means of holes 58 formed in the sidewall that can allow moisture penetration into the plug housing.